这几道数学题咋做啊
1.已知x+y+z=0,则1/（y²+z²-x²)+1/(z²+x²-y²)+1/(x²+y²-z²)= 2.若 3x-4y-z=0,2x+y-8z=0,则（x+y-z)/(x-y+z)= 3.已知a+b+c=0,且a,b,c均不为0,则a(1/b+1/c)+b(1/c+1/a)+c(1/a+1/b)=